A Ka-band push–push voltage-controlled oscillator (VCO) using a parasitic capacitance cancelling technique is presented. To enhance the oscillation frequency (fosc) and the tuning range (TR), the parasitic capacitance of the VCO is cancelled with negative capacitance (NC). To realise NC, a negative impedance circuit (NIC) is analysed and designed. Because of self-resonance frequency, NC is realised using an open-circuit stable NIC. For further enhancement of the fosc, the VCO is developed in a push–push operation. The proposed VCO is fabricated in a commercial 0.15 μm GaAs pHEMT process. It shows 12.4% of the TR at 31.6 GHz with 11 dBm output power. With the NIC, the centre frequency of the VCO increases from 20.1 to 31.6 GHz and the TR increases from 5.7 to 12.4% compared with a VCO without the NIC.